The mind is an extremely lazy dude, it doesn't want to do the hard work. Instead, it "thinks" about the process of getting that task done and finishing it. And while imagining about finishing the task, it actually starts thinking that the job is already done!
A strange sense of happiness is derived (but only temporarily) at that moment. Its as if the task is already finished and done (at least in the "thinking" realm) and we no longer have to worry about it and we forget about it momentarily.
But that's self delusional because the work isn't really done, the task isn't really finished, but we just mentally changed the red/amber flag to green and thus marked the mental "checkbox".
Then as reality sets in at regular intervals and keeps reminding us that its still not done, we go into depression. We feel doubly guilty for not doing the task in the first place, and also incorrectly marking it as "done".
The end result is that we never end up doing that task or even working on it. Because according to our mind, the task is already done! Why should I keep doing it again and again, it keeps asking.
Let's call the part of your mind that takes action (or does stuff) the processing department. The part that does the follow up at regular intervals depending on environment , external feedback, etc. is the reality check department. To put the problem in terms of this analogy, there is a grave lack of communication and co-ordination between these two departments.
Consider a large and bureaucratic govt. office where a file is stuck between two departments and isn't moving forward, what will you do then? As a high authority of the central government, you put pressure "from the above" on both departments to work properly and get the task done, isn't it? That's exactly what you must do in this case.
That high authority in this case is your super-ego or moral instinct in psychological terms. You may also call it your soul or higher-self or aatman if you believe in that kind of thing. That dude is probably sleeping somewhere in your mental realm right now, you must wake that dude up somehow and make him take control and finish this task.
The external world and the inner-self of a human have many similarities in how they work. Sometimes, tasks get done only when the highest authority intervenes!